Popcorn, the favourite snack of millions of Americans, can cause a potentially fatal health condition known as popcorn workers' lung, an alarmed public is discovering.

A love of microwave buttered popcorn caused a relatively healthy 53-year-old American to develop severe breathing problems. The cause of his illness was tracked down to the microwave popcorn he loved so much that he would inhale steam from the bag as it came out of the oven.

The link between the man's illness and popcorn was established by Dr Cecil Rose, who had been dealing with popcorn workers' lung for years as a consultant to the food industry. "I said to him this is a very weird question but bear with me, are you around a lot of popcorn?"

"His jaw dropped,' she told The New York Times, 'How could you possibly know that about me? I am Mr Popcorn. I love popcorn'," the patient replied. He had eaten buttery microwave popcorn at least twice a day for the past 10 years. When he broke open the bags, after the steam came out, he would often inhale the fragrance because he liked it so much," Dr Rose said. "That's heated diacetyl, which we know from the workers' studies is the highest risk."

Dr Rose found levels of diacetyl in the man's Colorado home after he made the snack were similar to those in microwave popcorn plants. She put him on a microwave popcorn-free diet. Six months after his diagnosis, the man has lost 50lb and his lung function has improved.

Synthetic butter or diacetyl inhaled as a vapour has damaged or destroyed the lungs of hundreds of workers in the food industry.